Update all LLVM documentation mentioning runtimes in LLVM_ENABLE_PROJECTS

We are moving away from building the runtimes with LLVM_ENABLE_PROJECTS,
however the documentation was largely outdated. This commit updates all
the documentation I could find to use LLVM_ENABLE_RUNTIMES instead of
LLVM_ENABLE_PROJECTS for building runtimes.

Note that in the near future, libcxx, libcxxabi and libunwind will stop
supporting being built with LLVM_ENABLE_PROJECTS altogether. I don't know
what the plans are for other runtimes like libc, openmp and compiler-rt,
so I didn't make any changes to the documentation that would imply
something for those projects.

Once this lands, I will also cherry-pick this on the release/14.x branch
to make sure that LLVM's documentation is up-to-date and reflects what
we intend to support in the future.

Differential Revision: https://reviews.llvm.org/D119351

1 files changed, 14 insertions, 6 deletions

diff --git a/README.md b/README.md
index 27cc755c93d0..15da30276508 100644
--- a/README.md
+++ b/README.md
@@ -63,17 +63,25 @@ This is an example work-flow and configuration to get and build the LLVM source:
 
         Some common options:
 
-        * ``-DLLVM_ENABLE_PROJECTS='...'`` --- semicolon-separated list of the LLVM
-          sub-projects you'd like to additionally build. Can include any of: clang,
-          clang-tools-extra, compiler-rt,cross-project-tests, flang, libc, libclc,
-          libcxx, libcxxabi, libunwind, lld, lldb, mlir, openmp, polly, or pstl.
+        * ``-DLLVM_ENABLE_PROJECTS='...'`` and ``-DLLVM_ENABLE_RUNTIMES='...'`` ---
+          semicolon-separated list of the LLVM sub-projects and runtimes you'd like to
+          additionally build. ``LLVM_ENABLE_PROJECTS`` can include any of: clang,
+          clang-tools-extra, cross-project-tests, flang, libc, libclc, lld, lldb,
+          mlir, openmp, polly, or pstl. ``LLVM_ENABLE_RUNTIMES`` can include any of
+          libcxx, libcxxabi, libunwind, compiler-rt, libc or openmp. Some runtime
+          projects can be specified either in ``LLVM_ENABLE_PROJECTS`` or in
+          ``LLVM_ENABLE_RUNTIMES``.
 
           For example, to build LLVM, Clang, libcxx, and libcxxabi, use
-          ``-DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi"``.
+          ``-DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"``.
 
         * ``-DCMAKE_INSTALL_PREFIX=directory`` --- Specify for *directory* the full
           path name of where you want the LLVM tools and libraries to be installed
-          (default ``/usr/local``).
+          (default ``/usr/local``). Be careful if you install runtime libraries: if
+          your system uses those provided by LLVM (like libc++ or libc++abi), you
+          must not overwrite your system's copy of those libraries, since that
+          could render your system unusable. In general, using something like
+          ``/usr`` is not advised, but ``/usr/local`` is fine.
 
         * ``-DCMAKE_BUILD_TYPE=type`` --- Valid options for *type* are Debug,
           Release, RelWithDebInfo, and MinSizeRel. Default is Debug.